The technique as such is well known in the art. A suitable tool comprises generally two separate tool parts which co-operate for producing said joint. A first tool part has the form of a punch which in a linear movement is driven in the direction of a co-axial second tool part in the form of a die with a die cavity at the bottom of which an anvil is arranged.
For making the joint the sheet formed members are positioned against the second tool part, the die, provided with movable die elements arranged sliding laterally on a support surface against the forces from a resilient element. The resilient element could be a ring made of an elastomer or a toroid formed metal spring surrounding the movable die elements.
The approaching punch impacts on the surface of one of the members to be joined. The material of the two members is first drawn into the die cavity and subsequently due to the interaction between the punch and the anvil at the bottom of the cavity laterally extruded thereby displacing the movable die elements outwardly creating in the sheet formed members a mushroom formed button which interlocks the members.
A tool of this type is e.g. shown in the document U.S. Pat. No. 5,946,782, published as WO97/02912 on 30 Jan. 1997, incorporated here by reference. FIG. 1 and FIG. 2 of that document show a tool for making joints between sheet-formed members 4, 5 comprising a first tool-part 1, 2, 3 with a punch 1 and a side pressing element 2 and a second tool-part 6, 11 provided with a support surface 9 from which an anvil 8 erects, a matrix 6′ comprising at least two matrix-parts 6 each having an upper active matrix surface 13 and being arranged around said anvil 8. Each matrix part comprises a portion 14 arranged sliding against said support surface 9 and being applied against the lateral surface of said anvil 8 by means of elastic means 7. Retention means 10 are arranged limiting the longitudinal movement of the matrix parts during the retraction of the punch 1. The upper active matrix surface 13, the portion 14 arranged sliding against said support surface 9 and said retention means 10 are arranged in that order from the top of the second tool-part 6, 11
The present invention is more particularly directed to such a second tool part in the form of a die with a die cavity at the bottom of which an anvil is arranged which second tool part is provided with a special type of resilient element.
The document EP12178068 shows a resilient or spring element which has the general form of a cage the surface of which has been provided with a number of slots having an acute angle, not necessary constant, relative the axis of the cage to form a corresponding number of long elastic arms or blades equally partitioned around the circumference. This resilient member produces the force against which the movable die elements are laterally displaced during the joining process. When the punch is retracted the movable die elements in contact with the deformed sheet members will be subjected to a vertical force trying to lift them up from their support surface. To prevent this, cooperating arrangements on the die elements and support elements on the die, parallel to the direction of the sliding movement of the die elements, have been provided. Thus, the resilient or spring element according to this document does not contribute to the retention of the movable die elements when the punch is retracted. As a consequence the cage formed spring element does not have to be fixed to the die body in the direction of the die axis.